The overall goal is to delineate the interactions among neuroendocrine mechanisms that regulate energy balance, hibernation, and reproduction. Circannual rhythms of each of these behaviors have been well described in the golden-mantled ground squirrel (Spermophilus lateralis), the model species in all experiments. Specific aims include: 1) characterizing metabolic fuel utilization during deep torpor and arousal; 2) determining the role of metabolic fuel availability in controlling hibernation bout length; 3) evaluating adipose tissue function at low temperatures of deep torpor; 4) specification of neural mechanisms that control metabolic fuel utilization during deep torpor and arousal; 5) describing the effect of size of adipose tissue stores on duration of torpor and frequency of arousal; 6) evaluating the relation between energy reserves and availability of specific metabolic fuels. Body weight related problems are a major medical concern. Much remains that we do not understand about the physiological mechanisms underlying food intake and body fat regulation. Deciphering how metabolic fuels are stored and mobilized in a model system like the golden-mantled ground squirrel could provide important insights into the regulation of these processes for mammals in general and humans in particular. Similarly, a better understanding of the mechanisms of reversible hypothermia is of potential medical import. The reduced metabolism, blood flow, etc. coincident with hypothermia may be very beneficial during many types of major surgery. Little is known at this time of the mechanism that allows ground squirrels and other hibernators to undergo prolonged hypothermia which is lethal to humans and other mammals. The proposed research addresses fundamental questions in regulatory biology and, thereby, affords the opportunity of establishing general principles applicable to all mammals.